Disorders of cognition are generally characterized by symptoms of forgetfulness, confusion, memory loss, attentional deficits, and/or, in some cases, affective disturbances. These symptoms may arise as a result of the general aging process and/or from organic brain disease, cerebrovascular disease, head injury, or developmental or genetic defects.
The general decrease in cognitive function which accompanies the aging process is well accepted. The same phenomenon has been observed and documented in many lower mammals, including those routinely employed in pharmacological testing programs for screening and predicting usefulness for particular drugs in higher animals, including humans.
Although disorders of cognition often accompany the general aging process, presenile and senile primary degenerative dementia are the most common accepted causes of mental deterioration in the elderly. It has been estimated that at least ten percent of persons over 60 years of age will eventually suffer severe mental deterioration. A much larger number will experience cognitive decline of sufficient severity to impede their activities.
Many of the symptoms of cognitive disorders, especially impaired memory, are associated with decreased acetylcholine synthesis and the impairment of cholinoreceptive neurons. In the hippocampus and cerebral cortex of patients suffering from primary degenerative dementia, for example, the level of the enzyme choline acetyltransferase (CAT) can be reduced by as much as 90%. (See Davies, et al, The Lancet, 1976 (Vol. 2) :1403; Perry, et al, J. Neurol. Sci., 34:247-265 (1977); and White, et al, The Lancet, 1977 (Vol. 1) :668-670).
Since CAT catalyzes the synthesis of acetylcholine from its precursors choline and acetyl coenzyme A, the loss of CAT reflects the loss of cholinergic, or acetylcholine-releasing, nerve endings in the hippocampus and cerebral cortex. There is abundant evidence that cholinergic terminals in the hippocampus are critically important for memory formation.
The cholinergic hypothesis suggests that drugs which restore acetylcholine levels or which mimic the action of acetylcholine (i.e., are cholinomimetic) are effective in correcting this deficit in neurotransmitter chemical and provide treatment of the memory impairment symptom of cerebral insufficiency. Considerable biochemical, pharmacological, and electrophysiological evidence supports the hypothesis that deficits in the cholinergic system underlie geriatric cognitive dysfunction. (See C. Peterson and G. E. Gibson, Neurobiol. Aging, 4:25-30 (1983)). Aged humans and nonhuman primates with decreased cognition show improved memory when they are treated, for example, with acetylcholinesterase inhibitors such as physostigmine. These agents increase the available supply of synaptic acetylcholine by inhibiting its hydrolysis.
Aminopyridines such as 3,4-diaminopyridine ameliorate age-related cognitive deficits by increasing the release of acetylcholine from presynaptic nerve terminals, thus increasing synaptic acetylcholine. (See H. P. Davis, et al, Exp. Aging Rest., 9:211-214 (1983)).
It has been known for some time that the natural alkaloid, muscarine, has the ability to act relatively selectively at autonomic effector cells to produce qualitatively the same effects as acetylcholine. Pilocarpine and arecoline (the methyl ester of 1,2,5,6-tetrahydro-1-methyl-3-pyridinecarboxylic acid), have the same principal sites of action as muscarine and acetylcholine and are thus classified as having "muscarinic" action. Although these naturally occurring alkaloids are of great value as pharmacological tools, present clinical use is largely restricted to the use of pilocarpine as miotic agent.
Recently it has been demonstrated that arecoline is effectively in ameliorating some of the symptoms of cognitive disorders in patients clinically diagnosed as having presenile primary degenerative dementia.